Both RF and DC SQUID's are known in the art, and have been proposed for use in volt meters, current meters, magnetometers, and amplifiers. These are very low intrinsic noise devices which have ultrahigh intrinsic sensitivity to magnetic fields. In particular, DC SQUID's have intrinsic energy sensitivities on the order of Planck's constant and show promise for providing very sensitive instruments.
A DC SQUID is a superconducting electronic device that is operated in a cryogenic environment. In its basic form, a SQUID acts as a two terminal element with a nonlinear current-voltage relationship. The exact current-voltage correspondence is dependent on the magnetic field applied to the device. Most of the applications for SQUID's make use of this magnetic flux dependence in order to read out signals. For example, reading out the voltage fluctuations in a current-biased, unshielded SQUID provides a method for detecting changes in the local magnetic field. In the usual device, a coil is placed near a current-biased SQUID so that a signal may be introduced as a current in the coil. The magnetic field generated by the current couples to the SQUID, influencing the voltage across it.
While an RF SQUID relies on RF frequency modulation to operate, a DC SQUID uses DC bias. The basis of any Josephson technology SQUID is a superconducting loop interrupted by one or more Josephson devices. In a particular case, the Josephson device can be a Josephson junction, a weak link, or a point contact junction. All of these have advantages and disadvantages, but the most suitable for the present invention is the Josephson tunnel junction. This junction is formed from a thin tunnel barrier (generally an insulating material) located at the interface of two superconducting electrodes. The tunnel barrier is sufficiently thin that Josephson currents can tunnel through it, and is typically 50 .ANG.. As is well known, the resistanceless Josephson current is a sinusoidal function of the phase difference of the electron wave function across the junction: EQU I=I.sub.0 sin .delta.,
where I.sub.0 is the critical current of the junction and .delta. is the phase difference. The time rate of change of the phase is proportional to the voltage drop across the junction: ##EQU1## where e is the electron charge, h is Planck's constant and V is the voltage drop across the junction. At currents less than the critical current I.sub.0 the junction exhibits zero resistance in the limit of low frequency, while at currents greater than I.sub.0 the phase difference continually increases, resulting in high frequency oscillations in the junction voltage. Unshunted junctions commonly show hysteresis. That is, if the junction current is increased to exceed the critical current, then the junction switches into the voltage state, and the current must be reduced to well below the critical current before the junction will switch back into the superconducting state. By shunting the junction with a small enough resistance, transitions between the superconducting and normal states may be accomplished in a nonhysteretic manner. This is known in the art, the condition for nonhysteretic behavior being that the parameter .beta..sub.c be less than unity, where: ##EQU2## where I.sub.0, R, and C are the junction critical current, shunt resistance, and capacitance. The single flux quantum .phi..sub.0 is given by ##EQU3## where e is the electronic charge and h is Planck's constant.
The state of the art in DC SQUID's is described in an article by M. B. Ketchen, which appeared in IEEE Trans. on Magnetics, Vol. MAG. 17, No. 1, January 1981, at page 387. Since these devices are well known, the following will be only a brief description of a DC SQUID. In it simplest form, two resistively shunted junctions are placed in a superconducting loop, and a bias current is introduced in such a way as to divide between the two junctions. Since the loop is superconducting, the current flowing around it can be described in terms of an electron wave function possessing both magnitude and phase. The Josephson equations relate the phase of the electron wave function across the junctions to the junction voltages. Single-valued-ness of the wave function requires that the phase difference around the SQUID loop be an integer times 2.pi.. Consequently, the oscillations in the phases of the two junctions of the DC SQUID are "coupled" by the superconducting loop. At all times, the phase is a well defined parameter which enables one to predict interference effects in multi-junction devices. If the phase around the loop is altered externally, such as by coupling a magnetic field into the superconducting loop, the critical current of the SQUID can be modulated. These modulations exhibit a period of .phi..sub.0 (2.07.times.10.sup.-15 webers) in the flux applied through the SQUID loop. The magnitude of the critical current modulations is a function of .beta.=2LI.sub.0 /.phi..sub.0, where L is the inductance of the loop. If the SQUID is biased at a constant current above the minimum critical current, a changing magnetic flux coupled into the loop can be detected by observing a modulation of the SQUID voltage. That is, the applied magnetic flux changes I.sub.0, causing the SQUID to switch to its voltage state then back to its superconducting state, etc. (shunted junctions are nonhysteretic). SQUID's are evaluated based on several parameters, including noise, the efficiency of the interface between the input coil and the SQUID (gaged by the coupling constant), and a figure of merit called the "energy sensitivity referred to the input coil". These terms are well known in the art, and are considered by designers using SQUID devices in geophysical, biomedical, and scientific applications. While the single SQUID circuit is suitable for many applications, such circuits will not provide extremely high bandwidth for use in very high frequency applications. For example, in the area of far infrared detection, an IF amplifier is required having a bandwidth of approximately 1 GHz. Conventional SQUID circuits cannot provide this bandwidth while having high sensitivity. If a SQUID IF amplifier could be provided, it would be a key component for a fully integrated millimeter wave receiver, such as a heterodyne receiver comprised of a tunnel junction array as a mixer, another array or high Q SQUID as a local oscillator, and a SQUID IF amplifier. Such a receiver would be the lowest noise receiver available at wave lengths of about 1 millimeter, and would be invaluable for detection of interstellar organic molecules and other astronomy uses.
In order to provide a high gain SQUID amplifier which has high bandwidth and sensitivity, it is proposed to cascade DC SQUID's on the same superconducting chip. In particular, a technique for coupling energy out of a SQUID stage is described, where essentially all of the current in any SQUID stage is transferred to the input of the next SQUID stage. Such a technique means that dependence on room temperature electronics is minimized and an entire cryogenic amplifier can be provided.
While cascading of amplifier stages can lead to circuit advantages, especially for high frequency, high bandwidth applications, the difficulty in providing a cascade is in effectively transferring current from one stage to the next without altering the desired current-voltage characteristics of individual SQUID stages. Generally, a substantial amount of bias current passes through the shunt resistors in the SQUID stages, and is lost. In the present invention, essentially all of the current which passes through the shunt resistors is transferred to the next stage of the amplifier, providing high sensitivity and gain. Further, there is no need for room temperature electronics to process the signals from one stage of the amplifier, prior to their use as the input to the next stage. A circuit for properly matching impedance and for keeping the impedance real to the highest frequency of the SQUID is used to effectively couple power from one SQUID stage to the next. Even though this circuit contains resistance, the bandwidth of the amplifier is not adversely affected. This is accomplished by an impedance matching network that allows essentially all of the current to be transferred to the next stage, including the currents through the shunt resistors, in a manner in which bandwidth is not lost.
Accordingly, it is a primary object of this invention to provide cascading of DC SQUID's on a single chip.
It is another object of this invention to provide an amplifier circuit comprised of DC SQUID stages, in which a minimum of off-chip circuit processing is required between the input to the amplifier and the output of the amplifier.
It is another object of this invention to provide an amplifier circuit comprising a plurality of stages of DC SQUID's, wherein the need for room temperature electronic circuits for signal processing is minimized.
It is another object of this invention to provide a fully integrated, low noise, broad bandwidth device comprised of a plurality of cascaded DC SQUID's.
It is another object of this invention to provide a DC SQUID amplifier having high gain and high bandwidth.
It is another object of this invention to provide a fully integrated, DC SQUID amplifier comprising a plurality of stages and feedback to improve input dynamic range and linearity.
It is another object of this invention to provide a technique for the effective transfer of current from one DC SQUID device to another DC SQUID device, where the transfer of current is accomplished over a very broad bandwidth.
It is a further object of this invention to provide a multiple junction DC SQUID having improved means for extracting power therefrom, wherein the extracted power has a wide bandwidth.
It is another object of this invention to provide at least two DC SQUID stages separated by a filter circuit having a real impedance over the frequency range desired, and wherein a bandwidth of approximately 1 GHz is obtained.
It is another object of this invention to provide a cascaded DC SQUID circuit where there is essentially complete transfer from one DC SQUID stage to the next, without detectable interactions between stages due to AC Josephson currents.